Pro-Israel groups have set sights on unseating this progressive lawmaker. Will they succeed?

In total, outside groups have spent more than $12m to support Bell as opposed to $3m for Bush.

After Bush moved up the street, Hoelting told the Guardian her questions were prompted by UDP advertising landing at her door.

Bush acknowledged that the UDP ads were having an impact.

Bush said that most voters accept her reasoning but it leaves some undecided. In contrast, she said her position on Gaza almost never gets brought up on the doorstep.

This leaves Bush all the more frustrated by the influence on the campaign of pro-Israel lobby money, much of which comes from billionaires who also donate to Republicans.

Other major funding has come from a long list of Republican donors including the billionaire hedge fund founders Jonathon Jacobson, Paul Singer and Bernard Marcus, the founder of Home Depot, all of whom are outspoken supporters of Israel.

Still, the timing of his switch has fuelled suspicions.

Earlier this week, the St Louis television station KDSK revealed a recording of a phone conversation made a year ago between the now-rival candidates in which Bell assured Bush he would not challenge her.

But days after the Hamas attack on Israel, Bell dropped out of the race against Hawley and announced he would run against Bush instead.

With Bush looking vulnerable, others have weighed in. The second largest spender in support of Bell after UDP is Fairshake, a group funded in good part by rightwing billionaires who also back Trump, such as Marc Andreessen, Ben Horowitz and the Winklevoss twins. Fairshake has spent more than $1m to defeat Bush.

Bush has also come under scrutiny for employing her husband to do security work, which she has defended as legal and not funded by her congressional office. The justice department said it was looking into the issue but a congressional ethics investigation concluded that the payments were legitimate.

Bush also has some important endorsements, including that of the father of Michael Brown, whose death at 18 at the hands of a Ferguson police officer 10 years ago fired up the Black Lives Matter movement.

Hannah Rosenthal, a member of Progressive Jews for St Louis, has been canvassing for Bush in Jewish neighbourhoods.

Bush said she was perplexed by accusations of antisemitism, given that she has spent her political career speaking up about racism.

Nonetheless, speaking up on Gaza has exacted a political price. Is it one worth paying?